1. Field of the Invention
The present invention relates to the fabrication of semiconductor devices and, more specifically, to a method of preventing cross-linking between multiple photoresists used in fabrication of the semiconductor device and simultaneously present on a semiconductor device structure under fabrication.
2. State of the Art
Photoresist (“resist”) layers are commonly used during semiconductor device fabrication to pattern a semiconductor substrate. A typical resist comprises a matrix or resin, a sensitizer such as a photoactive compound, and a solvent. When exposed to a specific wavelength of light, the photoactive compound initiates a photochemical reaction that causes the resist to cross-link. Images are patterned onto the semiconductor substrate by exposing portions of the cross-linked resist to the specific wavelength of light. The non-exposed and exposed portions are then developed to produce the desired pattern. Resists are also used to protect features during fabrication of the semiconductor device by covering those features with a resist layer.
Multiple resists may be used in the fabrication of a semiconductor device, with each resist formed on a portion of the semiconductor device. The resists may be simultaneously present on the intermediate semiconductor device structure. For example, a first resist may be used to protect features on the semiconductor device while a second resist may be formed over an active region of the semiconductor substrate to pattern the substrate. These resists may be formed in close proximity or contact with one another depending on the fabrication process being used. However, if the resists are in close proximity or contact, incompatible components of the resists may react with one another and cause the resists to react or cross-link. Cross-linking between resists is common between deep ultra-violet (“DUV”), I-line, and negative resists that are in close proximity or contact with one another. Cross-linking is especially common between I-line resists and negative resists. While cross-linking is common when at least one of the resists is unpolymerized, it may also occur when both of the resists are polymerized.
If resists in close proximity or contact cross-link with one another, they become unpatternable by conventional photolithographic processes because the resists are no longer selectively patternable. Therefore, in order to use two resists in close proximity or contact with one another, it would be desirable to be able to eliminate the potential for cross-linking between the resists.